The present invention relates to an electrophotographic method and apparatus for forming a color image on an image carrier such as a photosensitive body and, more particularly, to an electrophotographic method and apparatus for forming a color image by repeating a cycle of charging, exposure and development for a plurality of times.
Color recording using an electrophotographic technique has a long history, and various techniques have been proposed. Among them all, the most significant techniques which receive attention these days include a technique in which a light-emitting element such as a laser beam or an LED array is used to form an image on a photosensitive body, and a technique in which an optical system is used to write optical information digitized by a liquid crystal or an optical switching element utilizing the Faraday effect.
Why these techniques are the most significant for the color recording are following reasons. First, copy densities of individual color components conventionally are not reproduced faithfully due to a noncoincidence between spectral light intensity distributions of the individual color components which are caused by color separation of the original image and a nonuniform spectral sensitivity distribution of the photosensitive body. Conventionally, in order to resolve this problem of color reproducibility, the processing speed is determined in accordance with the lowest spectral sensitivity of the photosensitive body. However, this restriction can be eliminated by using the above-mentioned color recording techniques. Second, an S/N ratio can be improved since the optical signal is processed by an electronic circuit. Third, various applications such as electronic image processing (e.g., image information editing) are made possible upon incorporation of a computer.
In an electrophotographic apparatus using a method for writing digitized image data on a photosensitive body, reverse development is performed to visualize as a toner image that portion of the photosensitive body which is exposed by light beams. The reverse development method can decrease a load of a digital processing circuit and a optical scanning system with respect to scanning precision.
Basically, an image according to color electrophotography can be formed by repeating a cycle of charging, exposure and development for a plurality of times which are identical with number of colors of the image. The electrophotographic apparatuses are divided into two types: one type wherein chargers, exposure units and developing units are each disposed in a number corresponding to the total number of colors of the reproduced image to perform the cycle of charging, exposure and developing for each color upon one revolution of the photosensitive body; and another type wherein only developing units are disposed in a number corresponding to the total number of colors of the image and a single charger and a single exposure unit are also disposed around the photosensitive body such that charging and exposure for each color is completed upon rotations of the photosensitive body. The former system has a large construction, but provides a short recording time. Thus, this system is promising from the viewpoint of practical applications.
The most preferable and advanced arrangement of the multicolor recording apparatus as described above is basically illustrated in FIG. 1. This apparatus will be described with reference to FIGS. 1 and 2.
An original placed on an original table 1 is exposed by a known exposure optical system 2, and light reflected by the original is separated by a known tricolor separation filter 3. Separated light is incident on an image reading element 4 of a photoelectric transducer type which comprises a charge-coupled device (CCD) array called a solid-state imaging device or image scanner, or a photosensitive (e.g., silicon) array. Thus, three color components can be converted to corresponding electrical signals. These electrical signals are supplied to a memory/data processor 5. Thereafter, the signals are supplied through an output circuit 6 to optical image scanning units 9, 10 and 11, each of which comprises a laser beam array, a light-emitting diode (LED) array or a liquid crystal shutter array. An electrophotographic photosensitive body 8 as an image carrier charged by a charger 7 to a predetermined potential V1 is exposed using the optical image scanning units 9, 10 and 11. In this scanning/exposure operation, three optical outputs (red, blue and yellow in this embodiment since the tricolor separation filter is used) obtained in accordance with the color components separated by the tricolor separation filter 3 are scanned with beams 9a, 10a and 11a, respectively. Developing bias voltage VB higher than a potential VR1 of the exposure portion is applied to electrophotographic developing units 12, 13 and 14, respectively corresponding to the colors of the exposure light beams, so as to perform reverse development and hence form a multicolor image having three colors. The color image formed on the photosensitive body 8 is transferred by a transfer corona discharger 16 to a recording paper sheet P supplied from a paper supply unit 15. Thereafter, the paper sheet P thus transferred is separated by a separating unit 17 from the photosensitive body 8. The image formed on the paper sheet P is fixed by heat of a fixing unit 18, and the paper sheet is exhausted to an exhaust tray 19 outside the electrophotographic apparatus, thus completing the copying operation. Meanwhile, a developer which is not associated with the developing operation and which is left on the photosensitive body 8 is removed by a cleaner 21 after the photosensitive body is first discharged by a discharger lamp 20. Thereafter, the photosensitive body 8 is ready for the next copying cycle. According to the electrophotographic apparatus described above, an output from an external output device such as a computer and a word-processor can be connected to an input section 22 of the apparatus. Therefore, the apparatus can also be used as a multicolor printer for printing a multicolor image in accordance with color signals.
The present inventors have examined the conventional electrophotographic apparatus described above from various points of view and found the following problems.
The photosensitive body 8 charged by the charger 7 must maintain its charge thereon until it passes the third developing unit 14. However, in practice, the photosensitive body 8 can hardly comprise a photosensitive material which is uniformly charged for such a long period of time. Even if the photosensitive body 8 can comprise such a photosensitive material (e.g., pure selenium), the photosensitive material has a poor photosensitive property and has a spectral sensitivity restriction. Furthermore, even if the material has no restriction regarding spectral sensitivity, image quality is greatly degraded due to charge attenuation. In order to prevent such degradation of image quality, it is proposed that rechargers 23-a and 23-b for recharging the photosensitive body 8 prior to exposure for individual color components are arranged in front of the second and third developing units 13 and 14 so as to compensate a charge attenuation .DELTA.V from the photosensitive body 8. The necessary, stable potential for development is thus guaranteed by the rechargers 23-a and 23-b.
In this case, however, a potential distribution of the photosensitive body 8 is illustrated in FIG. 2 wherein the potential VR1 of a portion E exposed by the exposure beam 9a and the potential V1 of a nonexposed portion, as indicated by broken lines, respectively, in FIG. 2, change to potentials VR2 and V2, as indicated by solid lines, respectively, after recharging is performed. In this case, the already developed portion E must not be applied with the developer when the second and subsequent color reverse development cycles are performed. For this purpose, the electrostatic contrast value (VB-VR2) for development must be smaller than the developing sensitivity of the developer. However, in practice, the potential of the portion which is once exposed cannot be restored to the original potential, that is, the potential of the portion which is not exposed, even when the initial potential V1 of the photosensitive body 8 is kept constant. For this reason, the portion developed by the first developing unit 12 is developed again by the developing units 13 and 14, thus resulting in overlapping of colors. As a result, a desired color cannot be obtained.
This problem is based on the fact that satisfactory results can be obtained only when the photosensitive body 8 is entirely discharged and charged again. Therefore, latent image discharge light source must be arranged in addition to the rechargers 23-a and 23-b and an apparatus cannot be made compact as a whole. Repeated exposure of the photosensitive body 8 in the vicinity of the rechargers 23-a and 23-b is not preferred because it leads to fatigue of the photosensitive body 8. The present inventors have found that a fatigue phenomenon of a highly sensitive photosensitive body which comprises a selenium-tellurium alloy photosensitive material or an amorphous silicon photosensitive material was accelerated when the photosensitive body was repeatedly exposed.